1. Field of the Invention
The present invention relates to a positive resist composition for use in fabrication steps of semiconductors such as ICs, production of circuit boards for liquid crystals, thermal heads, etc., and lithographic steps of other photoapplications, and a pattern forming method using the composition. In particular, the invention relates to a positive resist composition suited for exposure with an immersion type projection exposure apparatus employing far ultraviolet light having a wavelength of 300 nm or less as an exposure light, and a pattern forming method using the composition.
2. Description of the Related Art
The trend toward miniaturization of semiconductor elements has lead to a decrease in the wavelength of exposure lights and an increase in the numerical aperture (NA) of projection lenses. As a result, an exposure apparatus which has an NA of 0.84 and employs an ArF excimer laser having a wavelength of 193 nm as a light source has been developed. As is generally well known, resolution and focal depth can be expressed by the following equations:(Resolution)=k1·(λ/NA)(Focal depth)=±k2·λ/NA2 
wherein λ is the wavelength of the exposure light, NA is the numerical aperture of the projection lens, and k1 and k2 are coefficients relating to the process.
For achieving enhanced resolution by further reduction in the wavelength, an exposure apparatus employing an F2 excimer laser having a wavelength of 157 nm as a light source is under investigation. Since materials for the lens to be used in the exposure apparatus and materials for resists are limited strictly, however, it is very difficult to stabilize the manufacturing cost of the apparatus or material or stabilize their quality. There is hence a possibility that an exposure apparatus and a resist having sufficient performance and stability cannot be provided within a required period.
As a technique for enhancing resolution of an optical microscope, a so-called immersion method, that is, a method of filling a liquid with a high refractive index (which may hereinafter be called “immersion liquid”) between the projection lens and the sample is known.
This “immersion” has the following effects. Assuming that the wavelength of the exposure light in air is λ0, the refractive index of the immersion liquid relative to that of air is n, the convergence half angle of light is θ and NA0=sin θ, the resolution and the focal depth when immersion is performed can be expressed by the following equations:(Resolution)=k1·(λ0/n)/NA0 (Focal depth)=±k2·(λ0/n)/NA02 
This means that the immersion produces the same effect as the use of an exposure light having a wavelength of 1/n. In other words, supposing that optical projection systems equal in NA are employed, the focal depth can be made n times larger by the immersion.
This is valid in any pattern profile. The immersion can therefore be used in combination with a super resolution technique such as the phase shift method or off axis illumination method which are being studied now.
Examples of an apparatus which has utilized the above-described effect for the transfer of fine image patterns of semiconductor devices are described in JP-A-57-153433 and the like.
Recent progress in the immersion exposure technique is reported in The Proceedings of The International Society for Optical Engineering (SPIE Proc.), 4688, 11 (2002), J. Vac. Sci. Technol., B 17 (1999), The Proceedings of The International Society for Optical Engineering (SPIE Proc.), 3999, 2(2000), etc. When an ArF excimer laser is used as a light source, pure water (refractive index at 193 nm: 1.44) is presumed to be most promising as an immersion liquid from the standpoints of safety in handling as well as transmittance and refractive index at 193 nm. A fluorine-containing solution is being studied as an immersion liquid for use in the exposure using an F2 excimer laser as a light source in consideration of balance between transmittance and refractive index at 157 nm. An immersion liquid satisfactory in view of environmental safety and refractive index however has not yet been found. Judging from the degree of the effect of the immersion and the maturity of resist, the immersion exposure technique will be first utilized in ArF exposure apparatuses.
Since the advent of a resist for KrF excimer laser (248 nm), chemical amplification has been employed as an image forming method of a resist for compensating a reduction in the sensitivity caused by light absorption. The image forming method, for example, using positive chemical amplification is a method of exposing a resist to light to cause decomposition of an acid generator in the exposed portions and generate an acid, subjecting the resulting resist to post-exposure bake (PEB) to utilize the acid thus generated as a reaction catalyst to convert an alkali-insoluble group into an alkali-soluble group, and removing the exposed portions by alkali development. As a chemical amplification type resist composition, resist compositions obtained by mixing two or more resins having specific structures are proposed, for example, in WO2005/003198 and JP-A-2002-303978. Although resists for ArF excimer laser using the chemical amplification mechanism are have recently become major resists, they need improvement because pattern collapse occurs when they are exposed to light through a mask with a very fine mask size.
It has been pointed out that when a chemical amplification type resist is exposed to immersion exposure, the resist layer comes into contact with the immersion liquid during exposure, resulting in deterioration of the resist layer or emission, from the resist layer, of components adversely affecting on the immersion liquid. International Publication WO 2004/068242 describes an example of a change in resist performance caused by immersing a resist for ArF exposure in water before and after exposure, while pointing out that this change is a problem in immersion exposure.
In an immersion exposure process, exposure using a scanning type immersion exposure apparatus needs movement of the immersion liquid keeping pace with the movement of a lens. If not, the exposure speed decreases, which may adversely affect on the productivity. When the immersion liquid is water, the resist film is desired to be hydrophobic and have good followability of water.
In addition, it is actually difficult to find an appropriate combination of a resin, photoacid generator, additive and solvent capable of satisfying the integrated performance of a resist. In forming fine patterns with a line width of 100 nm or less, even if the resolution performance is excellent, collapse of line patterns occurs, which may lead to defects during fabrication of a device. There is therefore a demand for overcoming this collapse of patterns and reducing line edge roughness which will otherwise disturb formation of uniform line patterns.
The term “line edge roughness” means that line patterns of a resist and edge of the interface of a substrate are shaped irregularly in a direction vertical to the line direction due to properties of the resist. Such patterns viewed from right above seem to have an edge with concavities and convexities (approximately from±several nm to±several tens of nm). Since these concavities and convexities are transferred to the substrate in an etching step, large concavities and convexities may deteriorate electrical properties, resulting in a reduced yield.